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Abstract

Concern over global climate change is widespread, but quantifying relationships between
temperature change and animal fitness has been a challenge for scientists. Our approach to this
challenge was to study migratory Pacific salmon (Oncorhynchus spp.), fish whose lifetime fitness
hinges on a once-in-a-lifetime river migration to natal spawning grounds. Here, we suggest that
their thermal optimum for aerobic scope is adaptive for river migration at the population level. We
base this suggestion on several lines of evidence. The theoretical line of evidence comes from a
direct association between the temperature optimum for aerobic metabolic scope and the temperatures
historically experienced by three Fraser River salmon populations during their river migration. This
close association was then used to predict that the occurrence of a period of anomalously high river
temperatures in 2004 led to a complete collapse of aerobic scope during river migration for a
portion of one of the sockeye salmon (Oncorhynchus nerka) populations. This prediction was
corroborated with empirical data from our biotelemetry studies, which tracked the migration of
individual sockeye salmon in the Fraser River and revealed that the success of river migration for
the same sockeye population was temperature dependent. Therefore, we suggest that collapse of
aerobic scope was an important mechanism to explain the high salmon mortality observed during their
migration. Consequently, models based on thermal optima for aerobic scope for ectothermic animals
should improve predictions of population fitness under future climate scenarios.